1. Field of the Invention
This invention relates to a continuous closed-loop system for pasteurizing liquid foods such as milk which maintains separation of the liquids during clean in place cycles for sanitizing the system and inhibits undesired mixing of the liquid food and cleaning and flushing liquids.
2. Description of the Prior Art
Pasteurization of liquid foods such as milk, fruit juice or soups requires that the liquid be raised to a sufficiently high temperature for a sufficient length of time so as to render the liquid safe for consumption for a specified period of time known as shelf life. Continuous pasteurization systems for milk must meet prescribed parameters set by governmental authorities for temperature and duration at that temperature. The United States Food and Drug Administration (FDA), for example, has developed the Pasteurized Milk Ordinance (PMO) which requires milk to be raised to a temperature of 162.degree. Fahrenheit (approximately 74.degree. C.) for a minimum of 16 seconds. Milk is no longer considered legal if untreated milk is later mixed with the pasteurized milk, and is unacceptable for sale and consumption if intermixed with cleaning liquids.
Milk pasteurization may be either a batch system or, more commonly, a continuous system. In a continuous system, it is necessary to deliver the milk to a holding tank after pasteurization. In such a system, it is necessary to prevent a buildup of vacuum in the milk pipeline which may inhibit or prevent the delivery of milk under satisfactory pressure. In such systems, it is also necessary to provide the capability of cleaning the internal components of the pasteurization system as skin or film, or burned-on milk may accumulate and afflict the quality of the pasteurized product. Modern production processes particularly favor clean-in-place systems. One valve particularly useful in inhibiting the reversal of the flow of treated or aseptic liquid and in permitting the system to be cleaned in place (CIP) is shown in U.S. Pat. No. 5,564,457.
FIGS. 1a and 1b, labeled "Prior Art", show a conventional layout for continuous processing system known in the art. For ease of understanding, the prior art system can be broken down into a raw milk intake section, a homogenizer section, a hot water supply section, a heat exchanger section, a holding section, a flow diversion section, a raw milk section and a control section.
In such a system, raw milk enters a balance tank which preferably provides a volume of milk held at a predetermined level to maintain a constant head for the milk supply. The raw milk is delivered either to a homogenizer, which serves as a timing pump, or once the system has reached and exceeded minimum pasteurizing temperature, through booster pump 1 to the regeneration section of the counterflow heat exchanger to elevate the heat of the entering raw milk by obtaining heat from the already treated milk and return this slightly heated milk to the homogenizer. The raw milk is then heated to the pasteurization temperature by heater of the heat exchanger section, typically from a steam energized hot water supply. After being heated in the heater section of the counterflow heat exchanger, the milk is passed to a holding tube where the milk is retained at an elevated temperature. The milk then passes to a safety thermal limit recorder (STLR) which monitors the heat of the milk leaving the holding tube. If the milk has not maintained sufficient temperature, then it is returned to the balance tank by the flow diversion device (FDD). If the temperature exceeds the minimum at the STLR, the milk is then considered aseptic or pasteurized.
The pasteurized milk then returns to the regeneration section of the heat exchanger for scavenging heat from the treated milk and raising the temperature of the raw milk as described above. After passing through the regeneration section, the pasteurized milk flows through a cooler section of the heat exchanger to lower its temperature, typically to about 35.degree. F. (about 1-20.degree. C.) After chilling, the pasteurized milk passes to a vacuum breaker valve to prevent backflow and past a backpressure valve. If any contaminants or raw milk has contacted the milk, or perhaps during a changeover from skim to 2% to homogenized or chocolate milk, the milk may be discharged to a drain or recycled to the balance tank for reprocessing. If the treatment is complete, then the final pasteurized milk is delivered to a holding tank for packaging and delivery.
In order to clean the system, the milk supply pipe must be physically disconnected and a clean in place liquid supply must be connected. That is to say, during start up when water is used as the system is heated to the desired temperature, a pipe arm swings over to replace the raw milk intake and is manually added to the balance tank before milk is introduced. Then, during the cleaning cycle, the milk pipe is physically disconnected and a different CIP pipe is connected to deliver cleaner (a sanitizer, alkaline or acid liquid cleaning solution) to the system. This same connection and disconnection process must be repeated during water flushes between products, e.g. between skim milk to 2% butterfat milk to homogenized (3.5% butterfat) milk. The need for manual connection and disconnection of separate pipes stems from the requirements of pasteurizing regulations that milk products and cleaning solutions be maintained separated without risk of crossover contamination.
Moreover, the short wash to scrub burn-on from the heater plates requires manual connection and disconnection of the pipes into the system. Each cycle of connection or disconnection risks introduction of debris or other contaminants into the liquid food, as well as consuming substantial time when the system could be productively operating. Typically, a short wash might have to be done 2 or 3 times in a production day, each period of decoupling and recoupling swing bends requiring about one hour of downtime in addition to cleaning time.
A need has thus developed for a liquid food pasteurizing system and method of operation whereby the need for physical disconnection of piping or other components in day to day operation may be eliminated.
There is additionally a need for a liquid food pasteurizing system capable of introducing water directly into the balance tank without reconnection.
There is further a need for a liquid food pasteurizing system which is capable of introducing and removing clean in place cleaners from the balance tank without reconnection.
There is additionally a need for a completely automatic liquid food pasteurization system which meets or exceeds existing governmental regulations without risk of intermixing raw liquid food with treated aseptic liquid food.
There is also a need for an automatic pasteurization system which avoids the need for routine manual intervention and reconnection to inhibit injuries or fully tightened connection.